MORPHOLOGY OF THE SEEDLING 29 



such a case as this the appropriateness of the name seed- 

 leaf is evident enough, one recognizes at sight the fact 

 that the cotyledons are actually the plant's first leaves. 

 In the bean the leaf-like nature of the cotyledons is not 

 so clear. They rise out of the ground like the squash 

 cotyledons, but then gradually shrivel away, though they 

 may first turn green and somewhat leaf-like for a time. 



In the pea (as in the acorn, the horse-chestnut, and 

 many other seeds) we have quite another plan, the under- 

 ground type of germination. Here the thick cotyledons 

 no longer rise above ground at all, because they are so 

 gorged with food that they could never become leaves ; 

 but the young stem pushes rapidly up from the surface 

 of the soil. 



The development of the plumule seems to depend some- 

 what on that of the cotyledons. The squash seed has 

 cotyledons which are not too thick to become useful leaves, 

 and so the plant is in no special haste to get ready any 

 other leaves. The plumule, therefore, cannot be found 

 with the magnifying glass in the unsprouted seed, and is 

 almost microscopic in size at the time when the hypocotyl 

 begins to show outside of the seed-coats. 



In the bean and pea, on the other hand, since the cotyle- 

 dons cannot serve as foliage leaves, the later leaves must 

 be pushed forward rapidly. In the bean the first pair are 

 already well formed in the seed. In the pea they cannot 

 be clearly made out, since the young plant forms several 

 scales on its stem before it produces any full-sized leaves^ 

 and the embryo contains only hypocotyl, cotyledons, and a 

 sort of knobbed plumule, well developed in point of size, 

 representing the lower scaly part of the stem. 



